ElectricLungAnatomy

Bronchial Anatomy

Brad H. Thompson, M.D., William J. Lee, B.S., Jeffrey R. Galvin, M.D. and Jeffrey S. Wilson, M.D
Peer Review Status: Internally Peer Reviewed

It should be pointed out that considerable anatomical variation may exist between individuals. The bronchial anatomy as described herein is illustrative of a typical bronchial pattern. The reader should be aware that often times, two or three bronchi may arise from a common trunk rather than having separate and discrete origins.

Trachea
The intrathoracic trachea is readily seen on computed tomography appearing as an air-filled tubular structure. The trachea begins at the level of the cricoid cartilage which generally is at the level of the sixth cervical vertebra. In adults, the trachea ranges 9 to 15 cm in length, terminating distally at the carina which represents the origins of the left and right mainstem bronchi. The trachea has a maximum transverse diameter of 16 mm, while sagittally, the trachea is narrower, having a maximal diameter of 14 mm. Because of the posteriorly directed horseshoe shaped cartilaginous rings, the posterior wall of the trachea tends to appear slightly flattened. This corresponds to the membranous portion of the tracheal wall.

The carina resides approximately at the level of the fifth thoracic vertebral body, and can be localized approximately at the same level as the sternal notch. On CT, the carina visually appears as a vertical cleft, representing the junction of the superomedial surfaces of the two mainstem bronchi.

The left mainstem bronchus (LMSB) measures approximately 4.5 cm in length compared to the right (RMSB) which measures approximately 2.5 cm in length. The shortness of the right mainstem bronchus is explained by the more proximal origin of the right upper lobe bronchus. Volumetrically, both mainstem bronchi taken together, have 40% more cross sectional area than the trachea. The left main stem bronchus leaves the trachea at a 135 degree angle. The right mainstem bronchus, more superiorly located, tends to be more vertically oriented, having a 155o angle of origin.

Right Upper Lobe Bronchi
Soon after its origin, the right mainstem bronchus (RMSB) gives rise to the right upper lobe bronchus which typically is directed superiorly and slightly laterally, having an almost 90o angle of incidence with the RMSB. The upper lobe bronchial trunk measures approximately 1 cm in length and approximately 1 cm in diameter. The trunk then gives rise to the segmental bronchi, B1, B2, and B3. The B1 bronchus supplies the apical segment of the right upper lobe and has a diameter ranging from 4 to 7 mm . On CT, this bronchus is typically imaged in cross section. The B2 bronchus, supplying the posterior segment has a more horizontal course relative to B1 but is nevertheless is readily visualized on CT. B3 supplies the anterior segment (S3) and like B2, has a generally horizontal course but proceeds somewhat inferiorly from its origin.

The right mainstem bronchus is considered to extend no farther inferiorly than the origin of the right upper lobe bronchus. The airway distal to the upper lobe bronchus is referred to as the bronchus intermedius (BI). BI generally averages 2 cm in length and terminates at the point at the origin of the right middle lobe bronchus.

Right Middle Lobe Bronchi
The middle lobe bronchial trunk measures approximately 12 mm in length, and 8 mm in diameter. The origin of the middle lobe bronchus marks the point of origin of the right lower lobe bronchus. From its origin off the anterior aspect of the bronchus intermedius, the right middle lobe bronchial trunk continues slightly inferiorly for a short distance before giving rise to the B4 and B5 segmental bronchi. On CT, both of these bronchi are almost routinely seen since they run almost parallel with the axial plane of section. B4 supplies the lateral segment while B5 supplies the medial segment (S5). The medial segmental bronchus has a slightly more oblique course than B4. In approximately 30-40% of the cases, B5 may be substantially larger than the lateral segmental bronchus.1

Right Lower Lobe Bronchi
The superior segmental bronchus (B6), may arise at, or above the level as the origin of the right middle lobe bronchus but more frequently arises slightly more distally . Regardless, B6 is the first branch off the lower lobe bronchus, and has a predominantly horizontal course making it readily identifiable on CT. The airway distal to B6 is referred to as the basilar trunk.

Because of their predominantly vertical orientation, the basilar segmental bronchi of the right lower lobe are routinely sectioned transversely on CT. It should be pointed out however, there is significant variation as to the points of origin of the basal segmental bronchi. The posterior and lateral basilar segmental bronchi typically arise from a common trunk. The medial basal bronchus (B7) has its origin inferior to B6. Oriented medially, B7 supplies the medial basal segment. Using their course as a guide, identification of the remaining three basilar bronchi is usually straightforward. Potentially arising from a common trunk, B8, B9, and B10, ultimately are seen coursing out to their respective segments. B9 + B10 is often referred to collectively as the terminal bronchus. B8 supplies the anterior basal segment; B9 courses laterally to supply the lateral basal segment; B10, directed predominantly posteriorly supplies the posterior basal segment.

Left Upper Lobe Bronchi
The origin of the left upper lobe bronchus occurs at a lower level than the origin of the right upper lobe bronchus. The left upper lobe bronchial trunk gives rise to the upper lobe and lingular segmental bronchi. Measuring 9 mm in length and approximately 12 mm in diameter, the left upper lobe bronchial trunk characteristically appears short but has a large diameter. The left upper lobe bronchial trunk divides giving rise to the ascending upper division (eventually giving rise to B1+2, and B3), and the descending lower division, which gives rise to the lingular segmental bronchi, B4 and B5. It should be pointed out that in the left upper lobe, the apical and posterior segments are combined, and as such are supplied by one bronchus, B1+2. The ascending upper division bronchus representing the common origin of the two segmental bronchi B1+2 and B3 differs substantially between individuals but on average is 1 cm in length and approximately 7 mm in diameter. The course of B1+2 has vertically and horizontally oriented components as bronchial rami divide to supply the apical posterior segment. The B3 bronchus will have a more horizontal course, similar to that seen on the right side. B3 supplies the anterior segment of the left upper lobe.

The lingular segmental bronchi are some of the most difficult segmental bronchi to visualize on CT. Their inconspicuousness is a result of their oblique course. If seen, B4 has a more horizontal course and supplies the superior lingular segment. B4 is superior to the more vertically oriented lingular bronchus (B5) which supplies the inferior lingular segment .

Left Lower Lobe Bronchi
The B6 (superior segmental bronchus) bronchus is similar to that seen on the right side having a typically horizontal course, supplying the superior segment. Reviewing (TABLE) shows that there are only 4 segments in the left lower lobe, compared to 5 on the right. The bronchial segment that supplies the medial basal segment on the right side ( B7 ) is not a separate entity on the left. As a result, the S7 and S8 segments are combined and supplied by B8, (anterior medial bronchus). Also because of the absence of B7, the left lower lobe basilar bronchial trunk typically measures slightly longer than the right lower lobe basilar trunk; while the overall diameter of the left lower lobe basilar trunk is similar measuring approximately 10 mm, the average length measures approximately 15 mm .

As is the case on the right side, the basilar segmental lower lobe bronchi course predominantly vertically, appearing in cross-section on CT. The direction and course make identification of the remaining lower lobe bronchi straight forward. Like their contralateral counterparts, B9, and B10 supply the lateral and posterior basal segments respectively, and may arise from a common trunk.

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